High performance nanostructured materials and methods of making the same

ABSTRACT

In accordance with the invention, nanostructured metallic materials having high tensile strength and increased ductility are prepared by providing a metallic material, deforming the metallic material to form a plurality of dislocation cell structures, annealing the material at a temperature from about 0.3 to about 0.7 of its absolute melting temperature, and cooling the annealed metallic material. The result is a nanostructured metal or alloy having increased tensile strength as compared with the corresponding coarse-grained material and substantially greater ductility as compared with nanostructured material made by conventional processes. Using this process applicants have made nanostructured alloys with tensile strengths in excess of 1.5 Gpa and ductility greater than 1 per cent strain-to-failure. They have also made nanostructured metals with tensile strength in excess of 400 MPa and ductility in excess of 50% strain-to-failure. The new materials are useful in a variety of applications such as rotors, electric generators, magnetic bearings, microelectromechanical devices and biomedical systems.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is continuation-in-part of U.S. patentapplication Ser. No. 09/970,402 filed by T. Weihs et al. on Nov. 3, 2001and entitled “High Performance Nanostructured materials and Method ofMaking the Same” which, in turn, claims the benefit of U.S. ProvisionalApplication Serial No. 60/237,732 filed by C. H. Shang et al. on Oct. 5,2000 and entitled “High Performance Nanostructured Materials and Methodsof Making the Same”. Application Ser. Nos. 09/970,402 and 60/237,732 areincorporated herein by reference.

GOVERNMENT INTEREST

[0002] The United States Government has certain rights in this inventionpursuant to Contract Number N00014-98-10600 supported by ONR andpursuant to Grant Number CMS-9877006 supported NSF.

[0003] This application also claims the benefit of U.S. ProvisionalApplication Serial No. 60/445,700 filed by E. Ma et al. on Feb. 7, 2003and entitled “High Tensile Ductility in a Nanostructured Metal”, whichis incorporated herein by reference.

FIELD OF THE INVENTION

[0004] This invention relates to nanostructured metallic materialshaving high strength and ductility and to methods of making them.

BACKGROUND

[0005] Nanostructured materials are of considerable interest due totheir unique mechanical properties and structural versatility. Materialswith grain sizes less than one micrometer can have significantlyimproved mechanical properties compared to conventional coarse-grainedmaterials. However, the starting materials, physical treatments, andfabrication conditions can significantly impact the performance ofnanostructured materials.

[0006] Nanostructured materials with high yield strength and hardnesshave previously been fabricated. However, poor ductility was observed,especially in high-strength intermetallic compounds. Nanostructuredintermetallics failed in the elastic regime under tensile stresses withvirtually zero plastic strain-to-failure at room temperature, severelylimiting their usage in industrial applications. This brittleness isattributed in part to flaws or porosity produced during fabrication.

[0007] Nanostructured materials are conventionally fabricated bysynthesizing various powders of nanometer size and then consolidatingthem, as by hot pressing, into bulk articles. However, this processingdoes not prevent the formation of micro-flaws or porosity.

[0008] One-step methods of synthesis, such as electro-deposition,crystallization of amorphous solids, and severe plastic deformation, canproduce materials without residual porosity, but these methods haveseveral disadvantages. First, nanostructured intermetallics made by onestep methods are extremely brittle. For example, nanostructured FeA1intermetallic had high strength of 2.3 GPa, but the material exhibitedsuch poor ductility that the strength was measurable only undercompression. Second, it is difficult to form bulk nanostructuredintermetallics because of the accumulation of deposition stresses.Forming bulk amorphous solids is technically complex and not practicalfor single-phase metallic materials. Single phase solids can be simplerto make, more stable, and may be desirable due to their magnetic,electrical, or optical properties. However, single-phase intermetallicshave not shown a combination of high strength and good ductility. Norhas this problem been solved by decreasing the grain size. Decreasingthe grain size is important for increasing strength, but grain sizeshould be decreased while reducing or eliminating the flaws (cracks) andporosity in the materials.

[0009] Similar problems are encountered with nanostructured(nanocrystalline) metals. Nanocrystalline metals, particularly thosewith grain sizes of less than 100 nanometers, have strengths exceedingthose of coarse-grained metals and alloyed metals. But conventionalnanostructured metals suffer severe loss of ductility. For example, purenanocrystalline Cu has a yield strength in excess of 400 MPa, which issix times higher than coarse-grained Cu. Its ductility, however, isgreatly reduced as compared with coarse grained Cu, with a tensileelongation to failure of less than about 5 per cent and an even smallerregime of uniform deformation.

[0010] Accordingly there is a need for a method for makingnanostructured metals and alloys having high tensile strength and goodductility.

SUMMARY OF THE INVENTION

[0011] In accordance with the invention, nanostructured metallicmaterials having high tensile strength and ductility are prepared byproviding a metallic material, deforming the metallic material to form aplurality of dislocation cell structures, annealing the material at atemperature from about 0.3 to about 0.7 of its absolute meltingtemperature, and cooling the annealed metallic material. The result is ananostructured metal or alloy having increased tensile strength ascompared with the corresponding coarse-grained material andsubstantially greater ductility as compared with nanostructured materialmade by conventional processes. Using this process applicants have madenanostructured intermetallics with tensile strengths in excess of 1.5Gpa and ductility greater than 1 per cent strain-to-failure. They havealso made nanostructured metals with tensile strength in excess of 400MPa and ductility in excess of 50% strain-to-failure. The new materialsare useful in a variety of applications such as rotors, electricgenerators, magnetic bearings, microelectromechanical devices andbiomedical systems.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The advantages, nature and various additional features of theinvention will appear more fully upon consideration of the illustrativeembodiments described in connection with the accompanying drawings. Inthe drawings:

[0013]FIG. 1 shows differential scanning calorimetry traces of as-rolledand annealed Hiperco Alloy 50HS, measured at a heating rate of 40° C.per minute;

[0014]FIG. 2 shows X-ray diffraction profiles for Hiperco Alloy 50HS:(a) as-rolled, and (b) annealed at 438° C. for five hours;

[0015]FIG. 3 is an image of a nanocrystalline FeCo-based intermetallicmaterial taken by transmission electron microscopy, showing grain sizeranging from tens to hundreds of nanometers of nanostructured material.The inset shows the discontinuous ring diffraction pattern. The clustersof diffraction spots are evidence for the growth of subgrains withlow-angle grain boundaries;

[0016]FIG. 4 is an image of a fracture surface for a nanocrystallineFeCo-based intermetallic with submicron dimples clearly showing thefracture is ductile;

[0017]FIG. 5 shows the results from room temperature tensile tests fornanocrystalline FeCo-based intermetallics;

[0018]FIG. 6 demonstrates room temperature strengths versus grain sizeof Hiperco Alloy 50HS samples;

[0019]FIG. 7 shows room temperature ductility versus grain size ofHiperco Alloy 50HS samples;

[0020]FIG. 8 shows Vickers hardness versus grain size for FeCo-basedintermetallics;

[0021]FIG. 9 shows Vickers hardness as a function of annealing time forHiperco Alloy 50HS;

[0022]FIG. 10 graphically illustrates engineering stress-strain curvesfor pure Cu in various forms;

[0023]FIG. 11 graphically represents tensile properties of pure Cu invarious forms;

[0024]FIGS. 12A and 12B show transmission electron micrographs of theevolution of the Cu microstructure;

[0025]FIGS. 13A and 13B show transmission electron micrographs of Cuafter different tensile strains; and

[0026]FIG. 14 is a schematic flow diagram showing the steps involved infabricating nanostructured materials having increased ductility.

DETAILED DESCRIPTION

[0027] In accordance with the invention, nanostructured metallicmaterials are provided having a unique combination of ultrahigh tensileyield strength and large tensile ductility. The nanostructured materialsmay be formed from any suitable metallic material including, but notlimited to pure metals (e.g., copper, nickel, iron), alloys, andintermetallic compounds (i.e. particular metallic chemical compoundsbased on a definite atomic formula). Nanostructured alloys andintermetallics are particularly advantageous for high strength, andnanostructured metals are particularly advantageous for high ductility.

[0028] The preferred nanostructured material has microstructures of lessthan one micrometer, typically grain size ranging from about 10nanometers to about 900 nanometers. The tensile yield strength of anexemplary nanostructured alloy can exceed about 1.5 GPa, while theplastic strain-to-failure ratio is at least 0.5% and preferably at least1%. An exemplary nanostructured metal has a tensile strength in excessof 400 MPa and a ductility in excess of 50% strain-to-failure. Theprecise mechanical properties desired can be achieved through controlledheat treatment.

[0029] The preferred nanostructured metallic materials are fully denseand essentially free of flaws and porosity. By “metallic materials” ismeant metals, alloys and intermetallic compounds. “Fully dense” refersto materials that have a density within 0.1% of their theoreticaldensity and “free of flaws and porosity” refers to materials that haveless than 0.1 vol % pores and are essentially free of cracks at grainboundaries. “Controlled heat treatment” or annealing of deformedstarting materials refers to heating the specimen in a controlledatmosphere with prescribed heat-up and ramp-down temperature rates andtime periods, resulting in the formation of small, nanometer scalegrains.

[0030] This description is divided into three parts: Part I describesthe method of the invention. Part II describes its application in thefabrication of preferred materials for high strength applications, andPart III describes its use in fabricating preferred materials of highductility.

I. Method of Fabricating Nanostructured Metallic Materials With HighStrength And Increased Ductility

[0031] Referring to the drawings, FIG. 14 is a schematic flow diagramshowing the steps in making nanostructured materials having highstrength and increased ductility. The first step shown in Block A is toprovide a metallic material such as a body comprising a metal alloy, anintermetallic compound or a pure metal.

[0032] The second step (Block B of FIG. 14) is to deform the metallicmaterial to form a plurality of dislocation cell structures.Advantageously the deformation comprises plastic deformation produced bya cold-rolling process, as described generally in U.S. Pat. No.5,501,747. The deformation advantageously achieves a reduction ratiotypically from between about 50% to about 95%. In a preferredembodiment, the deformation achieves the reduction ratio is at least80%, and preferably more than 90%. The deformation is preferably at roomtemperature but can also be done at lower temperatures (e.g. liquidnitrogen temperature).

[0033] The third step (Block C) is to anneal the deformed material. Theannealing temperature ranges from about 0.30 to about 0.70 of thematerial's absolute melting temperature for time periods ranging fromless than about one hour to more than about 100 hours. The annealing canbe conducted in a variety of atmospheres (e.g., hydrogen, argon, andnitrogen, or air).

[0034] Following annealing, the material is cooled (Block D). Thecooling rate can vary from less than about 1° C./minute to more thanabout 500° C./s. This process produces nanostructured materials havingultrafine grains with grain sizes from tens to hundreds of nanometerswithout noticeable grain growth when used at temperatures below theannealing temperature.

[0035] The steps of FIG. 14 provide a method of producing nanostructuredmaterials by forming grains of nanometer scale (less than a micron) inthe heavily deformed bulk articles through controlled heat treatments.Dislocation cell structures, ordering domains, and other chemical orphase defects act as driving forces to form nanometer-sized grains.Recrystallization and grain growth are employed to developnanostructured microstructures of diversified grain sizes The propertiesof nanostructured materials depend sensitively on the grain sizes.Varying grain sizes permits one to tailor the tensile strength andductility to meet particular needs of the material. The heat treatmentscan be conducted for a controlled period of time at a wide range oftemperatures to drive the recovery and recrystallization processes.

II. Fabrication of Preferred Materials For High Strength Applications

[0036] For applications requiring high strength with increasedductility, the metallic material is preferably an intermetallic compoundor alloy. Advantageous intermetallic compounds are single-phase alloyswhich form highly ordered crystalline materials. The preferredintermetallic compounds include the FeCo-based intermetallic HipercoAlloys 50 and 50HS, available from Carpenter Technology Inc. anddescribed in U.S. Pat. No. 5,501,747, which is incorporated by referenceherein. The chemical composition of the Hiperco Alloys in weight percentis: Alloy Element Composition in weight percent C 0.003-0.02  Mn  0.10max. Si  0.10 max. P  0.01 max. S 0.003 max. Cr  0.1 max. Ni  0.2 max.Mo  0.1 max. Co 48-50 V 1.8-2.2 Nb 0.03-0.5  N 0.004 max. O 0.006 max.

[0037] with iron as a balance. The preferred annealing temperature isgenerally between 0.30 and 0.70 of the absolute melting temperature(250° C.-950° C. for Hiperco Alloy 50HS) with an annealing time from1000 hours to several seconds. More preferred is an annealingtemperature in the range 0.37-0.53 of the absolute melting temperaturewith an annealing time from 50 hours to several minutes. The mostpreferred annealing temperature is from 0.39-0.44 of the absolutemelting temperature with the annealing time ranging from 20 hours toabout one hour. Recrystallizing plastically deformed ingots throughcontrolled heat treatments results in nanostructured metals, alloys, andhigh strength intermetallics that are fully dense and free of flaws orporosity.

[0038] Grain size can be limited to less than about one micrometer bycontrolling the annealing temperature and time. The controlled annealingprocess results in the release of energy as the defects in the materialare eliminated.

[0039]FIG. 1 is a Differential Scanning Calorimetric (“DSC”) scan ofHiperco Alloy 50HS showing the exothermic heat flow as a function oftemperature in comparing the “as-rolled” condition of the Hiperco Alloyto its condition after annealing. As shown in FIG. 1, the major recoveryand recrystallization process of the Hiperco Alloy 50HS material occursfrom between about 350 to about 705° C. Since FeCo 50HS melts at 1470°C., these temperatures correspond to 0.36 to 0.56 of the material'sabsolute melting temperature of 1743 Kelvin. A DSC scan is one of manytools known in the art that may be used to determine the temperaturerange of the recovery and recrystallization process for any givenstarting material. The process of cold-rolling deformation andsubsequent controlled recrystallization may be repeated one or moretimes to obtain still finer grains and higher mechanical strengths.

[0040] Advantageously the nanostructured materials contain niobiumcarbide (NbC_(x)) particles as retarders for grain growth. Compared withthe more than 99 wt % major phase, however, these second phase particlesoccupy only a small portion in volume. Microalloying elements such as Nbcontained in the nanostructured material preferably impede grain growthby nucleating particles at grain boundaries or by Nb atomspreferentially segregating to grain boundaries to act as a grainrefiner. The use of Nb in the nanostructured materials is a preferredmethod of maintaining the structural stability of the materials.

[0041] The fabrication for high strength applications may be moreclearly understood by consideration of the following specific examples:

Example 1 Nanostructured Materials With Tensile Strength Between 1.9 and2.3 GPA and Plastic Strain-to-Failure Between 1.3% and 5.5%

[0042] Hiperco Alloy 50HS (Co 48.68%, V 1.89%, Nb 0.31%, C 0.01%, Ni0.11%, Mn 0.04%, Si 0.03%, Cr 0.05%, and balanced with Fe) wascold-rolled to 152.4 micrometers with a rolling reduction of 92.6%. Thecold-rolled sheets were annealed in an ultrahigh purity hydrogenatmosphere at a temperature of 438° C. for five hours. The ramping ratewas 2-3° C./minute. To establish ordered intermetallic structures thatpossess superior soft magnetic properties, the cooling rate afterannealing was set at 1° C./min to 316° C. Based on the examinationresults of differential scanning calorimetric, cross-sectionhigh-resolution field emission electron microscopy, and transmissionelectron microscopy the nucleation period of the recrystallizationprocess was largely completed after the above heat treatment, and thecold-rolled alloys were successfully transformed into nanostructuredmaterials.

[0043] The grain sizes of the above processed nanostructured materialsranged from tens to hundreds of nanometers, with an average grain sizeof about 99 nanometers. The lower yield strengths ranged from 1.9 GPa tomore than 2.3 GPa depending on the test orientation with respect to therolling direction. The plastic strain-to-failure was 1.3% to more than5.0% depending on the loading direction. The in-plane Vickers hardnesswas as high as 6.4 GPa.

Example 2 Nanostructured Materials With Tensile Strength Between 1.3 and1.5 GPA and Ductility Between 11% and 18%

[0044] Hiperco Alloy 50HS alloy sheets were annealed at 650° C. for onehour. The other conditions were the same as those in EXAMPLE 1. Theaverage grain size in these samples was 287 nanometers. The lower yieldstrengths ranged from 1.3 GPa to more than 1.5 GPa depending on the testorientation with respect to the rolling direction. The strain-to-failurewas 11% to more than 18% depending on the loading direction.

Example 3 Nanostructured Intermetallic Materials With Fine Grain Sizeand High Ductility

[0045] Nanostructured intermetallics with an average grain size of 99 nmwere fabricated by annealing Hiperco Alloy 50HS at 438° C. in a hydrogenatmosphere for five hours (FIG. 3). Fractographic studies show that thedominant fracture mode for the fabricated nanostructured intermetallicsis ductile with submicron dimples (FIG. 4).

Example 4 Adjusting the Mechanical Properties of NanostructuredMaterials by Varying Grain Size and Heat Treatment

[0046] The mechanical properties of the nanostructured materials of theinvention are adjusted by varying the grain size and heat treatment ofthe materials. Decreasing the grain size (i.e., through use of a lowerannealing temperature) increases the tensile strength and decreases theductility (FIGS. 5 and 6). In contrast, increasing the grain size (i.e.,through use of a higher annealing temperature), decreases tensilestrength while increasing ductility (FIGS. 5 and 6). The lower yieldtensile strengths follow a similar Hall-Petch relationship, whethersamples are strained in the rolling or the transverse directions, with aslope of about 0.4 (FIG. 6). The ductility shows a peak around 500 nm,and decreases with reducing grain sizes (FIG. 7). The lowest ductilityobserved, about 1.3% plastic strain-to-failure, is significantly largerthan that of as-rolled materials, and much larger than any otherreported values for nanostructured intermetallics made by other methods.

Example 5 Vickers Hardness of the Nanostructured Materials

[0047] The hardness of the samples was measured on a LECO microhardnesstester (M-400) with Vickers indents (FIG. 8). At a temperature withinthe major recovery and recrystallization process, the Vickers hardnesswas found to increase logarithmically with the annealing time (FIG. 9),suggesting that the degree of recrystallization and grain growthincreases with time at a fixed annealing temperature.

Example 6 Additional Nanostructured Materials

[0048] The methods described in EXAMPLES 1-4 are applied to an aFeCo-based alloy consisting essentially of 48.78% cobalt, 1.92%vanadium, 0.05-0.31% niobium, 0.012% carbon, 0.1% nickel, balanced withiron cold-rolled to a reduction percentage of about 82.7% in thickness.

III. Fabrication of Nanostructured Materials of High Ductility

[0049] To achieve grain sizes as small as possible, very large amount ofcold work can be applied to a metal or alloy. Also, the cold work can bedone at subambient temperatures (e.g., by using liquid nitrogen coolingof the workpiece), especially for materials that dynamically recoverfast at room temperature. The desirable mechanical properties can beobtained by manipulating the grain sizes and their distributions in thenanocrystalline and ultrafine-grained regimes: an example is the use ofsecondary recrystallization of the already-recrystallized materialdescribed before. By doing this one can achieve a bimodal grain sizedistribution and the deformation in such a nonuniform microstructure canlead to ductility approaching that of a conventional coarse-grainedductile metal, without sacrificing much of the strength. In general, theprocess of FIG. 14 can therefore lead to materials with strength above1.5 GPa and yet with respectable ductility, or it can lead to materialswith ductility as high as >50% or more elongation to failure in tensionwhile maintaining a strength more than 3-5 times that of thecoarse-grained and annealed counterpart. The materials also exhibitstrain hardening that stabilizes the useful uniform tensile deformationto large plastic strains.

[0050] Application of the method of FIG. 14 to Cu results in a bimodalgrain size distribution, with micrometer-sized grains embedded inside amatrix of nanocrystalline and ultrafine (<300nm) grains. The matrixgrains impart high strength, as expected from an extrapolation of theHall-Fetch relationship. The inhomogeneous microstructure induces strainhardening mechanisms that stabilize the tensile deformation, leading toa high tensile ductility—up to 65% elongation to failure, and 30%uniform elongation. These results permit the development of toughnanostructured metals for forming operations and high-performancestructural applications including microelectromechanical and biomedicalsystems, as well as high strength, high conductivity metals.

[0051]FIG. 10 graphically illustrates engineering stress-strain curvesfor pure Cu in various forms. Curve A is annealed, coarse grained Cu.Curve B is Cu subject to room temperature rolling to 95% cold work (CW).Curve C is Cu subjected to liquid nitrogen temperature rolling to 93%CW. Curve D involves 93% CW and annealing at 180° C., 3 min. And Curve Eis 93% CW and annealing at 200° C., 3 min. Note the coexisting highstrength and large uniform plastic strain as well as the large overallpercentage elongation to failure for the Cu of curve E.

[0052] The pure copper is very ductile in its annealed andcoarse-grained form. It has an elongation to failure as large as 70%(curve A, FIG. 10), but a low yield strength (σ_(y)). Strengtheningthrough heavy cold work results in a material with a tensile curve thatpeaks immediately after yielding (curve B, FIG. 10). Such a trend ofstrengthening accompanied by a loss of ductility is generally true forCu and other metals processed in various ways.

[0053]FIG. 11 graphically represents the tensile properties of purecopper in various forms. The data are for Cu of conventional, ultrafineand nanocrystalline grain sizes and after cold rolling to variousdegrees of CW from applicants' own tests (filled black circles). Datapoints E, A, and B are from the corresponding curves in FIG. 10. Uniformelongation up to the peak in the engineering stress strain curve iscompared here, not only because it is a desirable property but alsobecause the overall percentage elongation to failure is often dominatedby localized deformation (necking) whose magnitude depends on the gaugelength used in the different experiments. The tough Cu developed here(E) is clearly separated from the general trend.

[0054] To provide nanostructured copper with high tensile ductility, ourprocessing starts by rolling the Cu at liquid nitrogen temperature to ahigh value of percentage cold work (CW). The resulting material has atypical heavily deformed microstructure with high densities ofdislocations in nanoscale networks, with some resolvable grains allbelow 200 nm in size. The use of the low temperature suppresses dynamicrecovery, allowing the density of the accumulated dislocations to reacha higher steady-state level than that achievable at room temperature.

[0055] Transmission electron micrographs of the microstructures afterrecovery annealing and recrystallization are shown in FIG. 12a, 12 b.These electron micrographs show the evolution of the Cu microstructure.FIGS. 12a and 12 b show the samples used to obtain the curves D and E inFIG. 10, respectively. After annealing at 180° C. for 3 min. (FIG. 12a),recovery has occurred, and the dislocation density is much reduced. Thevast majority of the grains are in the nanocrystalline/ultrafine range,with some recrystallized regions. Heat treating at 200° C. for 3 min.led to full recrystallization followed by secondary recrystallization(FIG. 12b). The recrystallized grains have well-defined, high-angleboundaries (FIG. 12b). The majority of the grains are in thenanocrystalline to ultrafine range. Meanwhile, abnormal grain growth(secondary recrystallization) has started to produce a volume fraction(˜25%) of coarser (1-3 μm) grains, some of which contain annealingtwins.

[0056] The engineering stress-strain curves corresponding to themicrostructures in FIG. 12 are compared in FIG. 10. After 93% CW atliquid-nitrogen temperature (curve C), the σ_(y) is much higher thanthat of the room-temperature-rolled Cu (95% CW; curve B). The elongationto failure is also significantly larger. After the 180° C. annealing,the σ_(y) decreased slightly for the recovered and partiallyrecrystallized grains, and the elongation to failure increased further(curve D). Such concurrent strengthening and toughening (in terms ofpost-necking elongation), as observed for both curves C and D whencompared with curve B, can be attributed to thenanocrystalline/ultrafine grains that reduce the size of nucleatingflaws and increase the resistance to crack propagation, leading to ahigher fracture stress. Micrographs of the fracture surfaces (not shown)indicate that ductile fracture through the nucleation and coalescence ofextremely fine microvoids was promoted. The radial unstable crack growthwas delayed, and the stabilizing triaxial stress state was maintained tolarger strains.

[0057] A marked improvement in uniform elongation was found concurrentwith pronounced strain hardening, without sacrificing much of thestrength, in material with the bimodal microstructure shown in FIG. 12b.The resultant stress-strain curve is shown in FIG. 10 (curve E). Thelarge densities of defects and cold-work energy stored during processingat liquid nitrogen temperature allowed copious nucleation duringrecrystallization at a low temperature, so that the vast majority of thematrix grains are kept in the nanocrystalline/ultrafine grain regime tohelp maintain the high strength of the ‘composite’ material. Meanwhile,the grains with sufficiently large sizes obtained through secondaryrecrystallization, at a volume fraction of 25%, produced pronouncedstrain hardening to sustain the useful uniform deformation to largestrains. Note that one should not restore strain hardening by allowinguniform growth of all grains or a large fraction of excessively growngrains, both of which would cause an additional unwanted drop in σ_(y).

[0058] The strain hardening is needed because the onset of localizeddeformation (necking instability, peak in FIG. 10) in tension isgoverned by the Considére criterion $\begin{matrix}{\left( \frac{\partial\sigma}{\partial ɛ} \right)_{\overset{.}{ɛ}} \leq \sigma} & (1)\end{matrix}$

[0059] where σ and ε are true stress and true strain, respectively. Thenanocrystalline and ultrafine matrix grains tend to lose the workhardening (left-hand side of equation (1)) quickly on deformation owingto their very low dislocation storage efficiency inside the tiny grains,especially when in presence of dynamic recovery. Such a high-strengthmaterial is therefore prone to plastic instability (early necking),severely limiting the desirable uniform elongation unless larger grainsof appropriate sizes and volume fractions are present.

[0060] Our strategy is to efficiently use the moderate population of thelarger grains to achieve a strain hardening rate much higher than thatwhich would be expected from curve A (FIG. 10, for uniform deformationunder uniaxial tension). We do this by taking advantage of the followingthree factors. First, these confined grains deformed in theinhomogeneous microstructure are under multi-axial stress states. Thereare complex strain paths and triaxial strain components, with very largestrain gradients. It is known that a complex stress state, complicatedstraining patterns and dislocation configurations, and high densities ofgeometrically necessary dislocations are all beneficial for promotinggrain refinement (or dislocation storage and strain hardening). Forexample, equal channel angular pressing (ECAP) uses similar conditionsto make nanostructured metals in an efficient way. For a metal such asCu, the non-uniform deformation over a length scale of the order of afew micrometers (FIG. 12b) is the realm of strain-gradient plasticitytheory, which predicts significant strain hardening owing to anexcessively large number of geometrically necessary dislocations thatare forced to be present to accommodate the large strain gradient.

[0061]FIGS. 12a and 13 b show transmission electron micrographs of Cuafter different tensile strains. The Cu sample is that shown in FIG. 12aafter 6% plastic strain. The upper left inset shows the selected-areaelectron diffraction pattern, and the lower right inset shows thehigh-resolution image of the boundary region between a larger,micrometer-sized grain (L) and one of the surrounding much smallerultrafine grains (S). A twin boundary (TB) is seen near the tip of the Sprotrusion into L, where twining is initiated. FIG. 13b shows the Cuafter the maximum uniform strain of ˜30%.

[0062] Second, <112> {11{overscore (1)}} twinning, as shown in FIG. 13aand the selected-area electron diffraction pattern in the upper leftinset, was observed unexpectedly after straining for 6% inside most ofthese larger grains. Deformation twins have not been observed for Cubefore except at high strain rates or low, sub-ambient temperatures, andactivation of such twins is known to require high stresses, especiallywhen the grain sizes are small. Additional observations by transmissionelectron microscopy (for example, the high-resolution image in FIG. 13alower right inset) show twin boundaries located preferentially near theprotrusions of the surrounding nanocrystalline/ultrafine grains into thesofter large grains, suggesting twinning initiation presumably due tostress concentration. The activation of the twinning mechanism suggeststhat these constrained larger grains plastically deform at highstresses, consistent with the high strength observed. In terms ofenhancing strain hardening, twinning is known to be highly effective inconventional Cu, owing to dislocation pileups at the twin boundaries(FIG. 13a). In nanocrystalline Cu, the interfaces generated betweentwinned segments can act as strong barriers to dislocation motion.

[0063] Third, the larger (softer) grains accommodate strainspreferentially. When the overall uniform elongation reached −30% (peakof curve E in FIG. 10), these larger grains had accumulated largenumbers of twin boundaries, dislocations and subgrain boundaries suchthat the microstructure was refined to a level similar to thenanocrystalline/ultrafine grained matrix, FIG. 13b). Afterwards, thepost-necking elongation is similar to that discussed for the sampleannealed at 180° C. (curve D in FIG. 10). Overall, the nanostructured Cuof curve E (FIG. 10), when compared with the coarse grained startingmaterial, represents an elevation of σ_(y) by a factor of 5-6 whilemaintaining comparable elongation to failure. The simultaneous highstrength and ductility, especially the very large uniform deformation atthe elevated strength, results in a notable gain in toughness (the areaunder the stress-strain curve). This is what sets this material apartfrom all previous treated copper materials, as demonstrated in FIG. 11.

[0064] To establish the reproducibility, three additional samples withor without the ECAP step were processed through similar CW and heattreatment. In all cases, coexisting high strength and ductility wereobserved. Further annealing beyond that shown in FIG. 12b causedadditional grain growth and larger uniform elongation, but with a largedecrease in σ_(y) and no gain in overall ductility owing to the decreaseof the post-peak elongation (compare curve A with curve D, FIG. 10).Attempts to start with the room-temperature-rolled Cu only managed aσ_(y) of˜100 MPa when elongation to failure reached −50%. Thisemphasizes the importance of the step at liquid nitrogen temperature,which stores large cold work energy that leads to a lowerrecrystallization temperature (compared with room-temperature rolling,the calorimetric recrystallization/grain growth peak temperaturedecreased by 60° C.) and favors copious nucleation over growth. Thismakes it possible to achieve the nanocrystalline/ultrafine grainedmatrix structure through recrystallization, thus affording room fortailoring the microstructure through controlled secondaryrecrystallization.

[0065] Our approach does not use uniform nanocrystalline grains, whichhave to rely on grain boundary deformation mechanisms (such as grainboundary sliding) to contribute significantly to ductility and stabilizethe plastic deformation through large increases in strain ratesensitivity. Experimental data so far (for example, FIG. 11) indicatethat at ambient temperature the increase in ductility provided by grainboundary sliding in small grains is either insufficient to compensatefor the loss of dislocation controlled ductility, or concurrent with amuch reduced strength.

[0066] Our idea of improving strain hardening may be used to derive goodductility from other nanocrystalline materials, where abnormal graingrowth is often observed. For example, after heating to a moderatetemperature nanocrystalline nickel was reported to exhibit a bimodalmicrostructure and pronounced strain hardening under certain deformationconditions. In addition to achieving a combination of strength andductility, our thermomechanical approach to the processing of bulksamples is also simpler than those processes required to produce uniformnanocrystalline grains; the latter processes are not only difficult orexpensive to implement, but also are difficult to keep free of artifactssuch as porosity and impurities. In fact, fracture due to sample flawsafter consolidation or deposition, together with plastic flowinstabilities such as necking and shear banding, are responsible for thevery limited strain to failure observed so far in nanocrystallinematerials.

[0067] The following specific examples further illuminate the nature andfeatures of the invention.

Example 7 Nanostructured Material of High Ductility

[0068] Pure Cu (99 99%) bar from a commercial source (ESPI) was firstprocessed by severe cold rolling, with liquid-nitrogen-temperature (LNT)cooling of the workpiece between consecutive rolling passes (−150 −C.and −100° C. before and after each pass, respectively). The degree ofLNT deformation is defined using per cent cold work,

%CW=(s ₀ −s)/s ₀×100%

[0069] where s₀ and s are the cross-sectional areas before and afterrolling. Some samples were processed through eight passes of equalchannel angular pressing (ECAP) at room temperature before rolling,while others were subjected directly to LNT rolling The ECAP step madeno obvious difference to the eventual microstructure and properties, asthe very large cold work energy stored at LNT controls the subsequentrecrystallization behaviour. Microhardness, as well as therecrystallization temperature and enthalpy storage measured in acalorimetric scan, levels off after about 90% CW

[0070] For mechanical property measurements, all the samples were cutand polished to a cross-section of 1 mm×1.8 mm, and a gauge length of 5mm (previously reported tensile tests of nanostructured metals typicallyused a gauge length in the range of 1-5 mm. Uniaxial tensile tests wereconducted at room temperature at an initial quasi-static strain rate of5×10⁻⁴ s⁻¹.

[0071] While preferred embodiments of the invention have been describedand illustrated, it should be apparent that many modifications to theembodiments and implementations of the invention can be made withoutdeparting from the spirit or scope of the invention. While theillustrated embodiments have been described utilizing a cold-rolling andcontrolled annealing process to produce nanostructured materials of hightensile yield strength and high ductility, it should be readily apparentthat other processes may be utilized (or steps added to the processes)to produce the unique nanostructured materials in accordance with theinvention. Any form of plastic deformation, particularly ashape-changing process (e.g., forging, swagging, extrusion etc.), thatresults in the generation of numerous dislocation structures withinexisting grains may be utilized. To facilitate formation of fully denseingots, the starting materials may be melted into a liquid state byvacuum induction melting or other suitable techniques, includingvacuum-based resistive furnaces, electron beam melting, reducedatmosphere melting, etc.

[0072] It can now be seen that the invention includes a method of makinga nanostructured material comprising the steps of providing the metallicmaterial, deforming the material to form a plurality of dislocation cellstructures, annealing the deformed material at a temperature from about0.3 to about 0.7 of its absolute melting temperature and cooling thematerial to produce the nanostructured material. The annealingtemperature is advantageously in the range 0.37-0.53 of the absolutemelting temperature and preferably in the range 0.39 to about 0.44. Theannealing time is advantageously from about 1000 hours to severalseconds and preferably from about 50 hours to several minutes. In oneform, the method involves annealing from about 0.39 to 0.44 of theabsolute melting temperature from about 20 hrs to about 1 hr. with timeand temperature to achieve a ductility of at least about 1% plasticstrain-to-failure and a tensile elastic yield strain of at least about0.5%.

[0073] The deforming step can advantageously comprise cold rolling witha thickness reduction ratio in the range from about 50% to about 95%.Advantageously the ratio is at least about 80% and preferably at leastabout 90%.

[0074] In another aspect, the invention comprises a nanostructuredmetallic material having a tensile yield strength of at least about 1.5GPa and a ductility of at least about 1 percent strain-to-failure. Thematerial is composed of microstructures with an average grain size of atleast 10 nanometers and preferably in the range from about 10 nanometersto about 900 nanometers. The material can typically have a tensileelastic yield strain of at least about 0.5% and a ductility from about 1to about 18% plastic strain-to-failure. It can have ductility typicallyfrom between about 1.3 to about 5.5 percent plastic strain-to-failure,and it can have a Vicker's hardness typically of about 5.5 to about 10Gpa.

[0075] In yet another aspect, the invention comprises a nanostructuredmetallic material having a tensile yield strength of at least about 400MPa and a ductility of at least about 5% strain-to-failure. Theductility can be 30 percent or higher. In preferred metals the strengthcan be in excess of three times that of the conventional coarse-grainedmetal and the ductility can be in excess of 50 percentstrain-to-failure.

[0076] The deforming can comprise cold working a metal at reducedtemperatures, e.g. liquid nitrogen temperature. The cold worked metalcan then be heat treated to recrystalization and secondaryrecrystallization.

[0077] It is understood that the above-described embodiments areillustrative of only a few of the many possible specific embodiments,which can represent applications of the invention. Numerous and variedother arrangements can be made by those skilled in the art withoutdeparting from the spirit and scope of the invention.

What is claimed:
 1. A method of making a nanostructured metallicmaterial comprising the steps of: providing a metallic material;deforming the metallic material wherein a plurality of dislocation cellstructures are formed; annealing the metallic material at a temperaturefrom about 0.3 to about 0.7 of its absolute melting temperature; andcooling said metallic material to produce nanostructured material. 2.The method of claim 1, wherein said temperature is from about 0.37-0.53of its absolute melting temperature.
 3. The method of claim 1, whereinsaid temperature is from about 0.39 to about 0.44 of its absolutemelting temperature.
 4. The method of claim 1, wherein said temperatureis at least about 350 degrees Celsius.
 5. A method of adjusting thetensile strength of a nanostructured material comprising: providing ametallic material; deforming the metallic material wherein a pluralityof dislocation cell structures are formed; annealing the metallicmaterial at a temperature from about 0.30 to 0.70 of its absolutemelting temperature for a time from about 1000 hours to several seconds;and cooling the metallic material.
 6. A method of adjusting theductility of a nanostructured crystalline material comprising the stepsof: providing a metallic material; deforming said metallic material sothat a plurality of dislocation cell structures are formed; annealingsaid metallic material at a temperature from about 0.37 to 0.53 of itsabsolute melting temperature for a period of time from 50 hours toseveral minutes; and cooling said metallic material after said annealingstep.
 7. A method of adjusting the ductility of a nanostructuredcrystalline material comprising the steps of: providing a metallicmaterial; deforming said metallic material so that a plurality ofdislocation cell structures are formed; annealing said metallic materialat a temperature from about 0.39 to about 0.44 of its absolute meltingtemperature for a period of time from about 20 hours to about 1 hour,wherein the temperature and time are selected to achieve a ductility ofat least about 1% plastic strain-to-failure and a tensile elastic yieldstrain of at least about 0.5%; and cooling said metallic material aftersaid annealing step.
 8. The method of claim 5 wherein said deformingstep further comprises cold rolling said metallic material with athickness reduction ratio in the range from about 50% to about 95%. 9.The method of claim 8 wherein said thickness reduction ratio is at leastabout 90%.
 10. The method of claim 8 wherein said thickness reductionratio is at least about 80%.
 11. A nanostructured metallic materialhaving a tensile yield strength of at least about 1.5 GPa and aductility of at least about 1 percent strain-to-failure.
 12. Thenanostructured material of claim 11, further comprising microstructureswith an average grain size ranging from about 10 nanometers to about 900nanometers.
 13. The nanostructured material of claim 11, furthercomprising microstructures with an average grain size of at least 10nanometers.
 14. The nanostructured material of claim 11 having a tensileelastic yield strain of at least about 0.5% and a ductility from about 1to about 18 percent plastic strain-to-failure.
 15. The nanostructuredmaterial of claim 11, wherein said ductility is from between 1.3 toabout 5.5 percent plastic strain-to-failure.
 16. The nanostructuredmaterial of claim 11, wherein said the nanostructured material has aVicker's hardness of about 5.5 to about 10 GPa.
 17. Nanostructuredmagnetic materials, wherein the materials are cold-rolled and annealedat a temperature ranging from about 350 to about 705 degrees Celsius,have a room temperature yield strength in excess of about 1.2 GPa andtensile ductility in excess of about 1% plastic strain-to-failure. 18.The nanostructured magnetic materials of claim 17, wherein the materialsconsist essentially of about 0.003% to about 0.02% C, no more than about0.10% Mn, no more than about 0.10% Si, no more than about 0.01% P, nomore than about 0.003% S, no more than about 0.1% Cr, no more than about0.2% Ni, no more than about 0.1% Mo, from about 48 to about 50% Co, fromabout 1.8 to about 2.2% V, from about 0.03 to about 0.5% Nb, no morethan about 0.004% N, and no more than 0.006% O, and iron as the balance.19. The nanostructured magnetic materials of claim 17, wherein saidmaterials consist essentially of 48.78% cobalt, 1.92% vanadium, 0.06%niobium, 0.012% carbon, 0.1% nickel, balanced with iron.
 20. Ananostructured metallic material having a tensile yield strength of atleast about 400 MPa and a ductility of at least about 5 percentstrain-to-failure.
 21. The nanostructured material of claim 20 whereinthe ductility is at least 30 percent strain-to-failure.
 22. Thenanostructured material of claim 20 wherein the metal comprises copper.23. The nanostructured material of claim 20 wherein the metal consistsessentially of copper.
 24. The nanostructured material of claim 20wherein the nanostructured metal has a strength in excess of 3 times thestrength of the conventional coarse-grained metal and a ductility inexcess of 50 percent strain-to-failure.
 25. The method of claim 1wherein the nanostructured metallic material is metal and thedeformation comprises cold working the metal.
 26. The method of claim 25wherein the metal is cold worked at liquid nitrogen temperature.
 27. Themethod of claim 25 wherein the cold worked metal is heat treated torecrystallization and secondary recrystallization.